The Relationship between Primary Hyperparathyroidism and Thrombotic Events: Report of Three Cases and a Review of Potential Mechanisms.

We have described three uncommon cases of patients who presented with clinical thrombotic events (stroke, pulmonary embolism and deep venous thrombosis) during the course of a hypercalcemia-induced hypercoagulable state. After thorough investigation, the diagnosis of primary hyperparathyroidism - due to a parathyroid adenoma - was established in all cases. The association between hypercalcemia and venous or arterial thrombosis has been previously described; however, relevant data are still insufficient. The existing evidence in the field was reviewed and the interesting underlying pathophysiologic mechanisms were also discussed. Further studies are required to shed more light on the unusual, still intriguing relationship between calcium and thrombosis.


INTRODUCTION
Hypercalcemia, a common electrolyte disorder in the clinical setting, is most frequently associated with primary hyperparathyroidism and malignancy 1 . Other, less common, causes include immobilization, drugs such as thiazide diuretics, granulomatous diseases (sarcoidosis, tuberculosis, lymphomas, among others) and augmented intake or absorption 2 . Primary hyperparathyroidism (PHPT) is characterized by the unregulated overproduction of parathyroid hormone (PTH) resulting in irregular homeostasis of calcium. The disease has been reported to have a prevalence of nearly 21 cases per 100.000 person-years, a mean age of diagnosis between 52 and 56 years, while it is considered significantly more common among females (femaleto-male ratio of 3:1) 3 . With regards to its etiology, the majority of the cases (about 85%) are attributed to a single adenoma of the parathyroid glands. In the remaining 15%, the disease is associated with either multiple adenomas or hyperplasia, while only infrequently (less than 1%), a parathyroid carcinoma can be the cause of the disease 4 .
The clinical spectrum of PHPT includes manifestations from multiple systems such as skeletal (osteitis fibrosa cystica), renal (kidney stones, polyuria), gastrointestinal (constipation, abdominal pain, peptic ulcers) and neurological (lethargy, confusion, stupor, and coma) 5 . PHPT has been also linked with cardiovascular disease (CVD), as well as, increased morbidity and mortality 6 . Uncommon cardiovascular manifestations of PHPT include left ventricular hypertrophy, cardiac and vascular function abnormalities and increased carotid intima-medial thickness (IMT) 7 . Additionally, other CVD risk factors such as arterial hypertension, diabetes mellitus, increased Body Mass Index (BMI) and dyslipidemia have been described to correlate with PHPT 8 . The association between PHPT and thrombotic events, which has been previously reported in the literature, still little is known in terms of the pathophysiological mechanisms that connect these entities. It still remains uncertain whether the pathophysiological background of thrombosis in PHPT is related to prothrombotic factors acting indirectly (e.g. increased prevalence of risk factors for CVD), directly (causing disturbances of the hemostatic system) or even a combination of both. In this article, we have described three patients who presented with thrombotic events (stroke, pulmonary embolism and deep venous thrombosis, respectively) and after thorough investigation the diagnosis of PHPT -due to a parathyroid adenomawas established in all cases.

Case 1 (Stroke)
A 63-year-old, female, left-handed patient presented to the Emergency Room with expressive aphasia for the last 5 hours. Her past medical history was unremarkable and she was not receiving any medications on a regular basis. Apart from aphasia, physical examination on presentation was normal. Blood samples were taken for initial laboratory evaluation and an urgent brain Computed Tomography (CT) scan was also A second CT scan excluded intracranial hemorrhage and a Magnetic Resonance Imaging (MRI) brain scan after 48 hours showed another acute infarct at the anatomical area of the right pons. Unfortunately, due to technical reasons, we were not able to perform a CT angiography of the intracranial vessels. Further investigation revealed high serum PTH levels (11.1 pmol/l, 1.58-6.03), marginally low serum phosphorus levels (2.7 mg/dl, 2.7-4.5), vitamin D insufficiency [25(ΟΗ)D3 22 ng/ml, 30-100] and elevated urinary calcium excretion (325 mg/24h, 100-250) ( Table 1).  (Table 1). Parathyroid sestamibi scintigraphy was indicative for an adenoma in the left lower parathyroid gland. CT scans of the thorax and abdomen, as well as mammography were negative for malignancy. The combination of long-term arthralgia and DVT raised the suspicion of an underlying autoimmune disorder. Multiple autoantibodies, including antiphospholipid, anti-cyclic citrullinated peptide (anti-CCP), anti-neutrophil cytoplasmic (ANCAs), antinuclear (ANA) and anti-double stranded DNA (anti-dsDNA) were measured, but were all negative.
In the end, a lip biopsy confirmed the diagnosis of primary Sjogren's Syndrome (SS). The patient was subjected to parathyroidectomy, and histological examination of the removed lesion revealed a benign parathyroid adenoma. After surgery, both calcium and PTH levels returned to normal. She remains under the care of the rheumatology team for monitoring recently diagnosed SS.

DISCUSSION
Back in 1973, Hilgard, studying a rat model, supported that acute hypercalcemia induces a hypercoagulable state 9 . Motivated by these findings, he suggested that hypercalcemia may also favor thrombus formation in vivo. However, to the best of our knowledge, recent clinical data supporting this view remain scarce. In the early 1970's, Bostrom and Alveryd studied 170 patients with hypercalcemia, of whom 9 were diagnosed with stroke 10 . The same investigators  randomly  evaluated  2268  patients  for  hypercalcemia  and  found  12 with hyperparathyroidism, of whom 3 had lately been diagnosed with stroke. In the mid 1980's, Gorelick and Kaplan reported 6 patients with hypercalcemia among a total number of 502 stroke patients 11 . All of them had elevated PTH levels, but parathyroid adenoma and hyperplasia were detected in only 2 cases. Angiography was performed in 3 cases, demonstrating distal branch artery occlusion in 2 patients and distal branch narrowing, probably attributed to vasoconstriction, in the other case. Given the relatively low frequency of hyperparathyroidism in individuals sustaining a thrombotic event, it is likely that other contributory factors are required, and hypercalcaemia simply "facilitates" thrombotic vascular occlusion through various mechanisms (Figure 1) further discussed below. The pathophysiologic background that connects calcium and thrombosis is not completely understood, however, several potential mechanisms have been proposed. It is believed that calcium triggers vascular smooth muscle leading to vasoconstriction 12,13 activates several factors of the clotting system and also boosts platelet aggregation 14 . Additionally, renal mechanisms for the reabsorption of sodium and water are disturbed by the hypercalcemic state, and non-compensated polyuria due to nausea and anorexia can lead to dehydration and hypercoagulable state 3 . Furthermore, elevated calcium concentrations can be potentially responsible for cytotoxic effects that stimulate cell death and result in thrombosis 15 . A case-control study of 23 patients with PHPT showed that platelet count, FVII, FX activities, and D-Dimer levels were significantly increased in patients compared to controls 16 . In a later study, tissue plasminogen activator inhibitor-1 (PAI-1) and tissue plasminogen activator (t-PA) were also found to be increased, while the tissue factor pathway inhibitor (TFPI) was decreased in patients with PHPT International Journal of Hematology Oncology and Stem Cell Research ijhoscr.tums.ac.ir compared to healthy controls 17 . Elevated PAI-1 has been also reported in a cohort study of patients with PHPT and was found to correlate with PTH levels 18 . The presence of other mechanisms that act independently of hypercalcemia and promote atherosclerosis in patients with PHPT should also be considered. For example, recent data indicate that raised PTH levels are linked with high risk of cardiovascular disease in patients with chronic kidney failure even in the absence of elevated calcium and phosphate levels 19 . It seems that the key point which connects PTH and atherosclerosis in these cases is the vascular and valvular calcification, while reduction of PTH levels with cinacalcet has been proved to be protective against cardiovascular disorders 20 . The possibility that PHPT may synergically promote thrombosis when interacting with other prothrombotic factors such as mutations (as in case 2) or autoimmune diseases (as in case 3) cannot be excluded (Table 2).